1. Field of the Invention
The present invention relates to subsurface safety valves for well tubing.
2. Description of the Prior Art
In the prior art, safety valves have been mounted with well tubing to control subsurface well pressure and prevent blowouts which caused pollution of the environment, particularly on offshore wells, as well as damage to expensive equipment by explosion, fire and the like. Sand and other abrasive materials in the oil passing through the tubing caused wear and deterioration of the valves rendering them defective and ineffective, and necessitating replacement of the defective valves.
Certain of the prior art safety valves were mounted in well tubing, requiring the well to be shut-in and the tubing removed to replace the defective valve. Removal of the well tubing was expensive and time-consuming and caused reduced production capacity due to the well being shut-in. Further, there was always a risk that the shut-in well might "sand up" or become blocked, requiring reworking of the well before production could be resumed.
Other prior art safety valves such as those known by the name "STORM CHOKES", and those of U.S. Pat. Nos. 3,236,255; 3,411,584; 3,398,762; 3,279,545; and, 3,310,114 were removably mounted with the well tubing and placed in the tubing by wireline and fishing tool or other equipment. Certain of these removable prior art safety valves closed when a pressure surge in the well tubing caused a predetermined pressure drop across a bean or other structure in the tubing, while others closed in response to the pressure surge overcoming a counter-pressure from a predetermined quantity of liquid or gas housed in a portion of the valve housing. Other prior art replacement safety valves had hydraulic operators mounted therewith to control the operation thereof. The hydraulic operator occupied needed space in the well bore and reduced the size of the throughbore which was formed in the replacement valve, restricting the flow of fluid through the well tubing.
With these types of prior art valves, it was difficult to determine whether the valve was defective without removing the valve from the well tubing and inspecting it. Removal and inspection of the valves was expensive and time-consuming, and a risk of blowout was present while the valve was removed for inspection. Also, the prior art safety valves were often ineffective when the wells were producing at or near capacity, being unable to experience any pressure surge due to the high flow rate in the tubing.